Over the years I had C. Sharps build me rifles in 45-110, 45-100, and 45-90, all different configurations; 1 heavy buffalo era, 2 target guns with single triggers and pistol grip stocks. My intent was to shoot PPB's so I wanted the extra length cases to accommodate a grease cookie with the PPB seated relatively deep in the case. They all shot great with PPB's but now in 45 caliber, I am back to shooting a 45-70 with GGB's and can't see a reason to mess with the longer cases. If I were shooting LR matches then probably a 45-90 but I am not.

Ron Smith of RKS Enterprises changes barrels or rechambers or rebores numerous single shot rifles.

I would keep the 45-120 barrel and have a second 45-70 barrel made for the action.

It is not that difficult to switch the barrels and shoot both .45 calibers with the Sharps action.

When I have multiple barrels for a rifle I also have a separate forearm properly bedded for each barrel as well.

You then need a barrel wrench and a padded vise to change (unscrew) the barrel after removing the extractor first.